Organic biomass fractionation process

ABSTRACT

A method for fractionating fibrous biomass comprising cellulose, hemicellulose and lignin components to separate said lignin, cellulose and hemicellulose from one another comprises:  
     (a) shredding said fibrous biomass;  
     (b) concurrently with or subsequent to said shredding, contacting said biomass with an aqueous solution of a nitrate ion source at a concentration of about 0.1-0.3% at a temperature in the range of about 60° to about 80° C. to initiate nitration of the lignin component of said biomass;  
     (c) submerging said partially nitrated biomass in an aqueous solution of a nitrate ion source in the presence of an aluminum compound at a temperature within the range of about 75-100° C. for a time sufficient to complete the nitration of said lignin component;  
     (d) contacting the nitrated biomass produced in step (c) with an alkaline extraction liquor comprising NH 4 OH at an initial concentration sufficient to solubilize said nitrated lignin component and said hemicellulose component from said cellulose component of said biomass;  
     e) recovering said cellulose from said extraction liquor containing said solubilized nitrated lignin and hemicellulose components, wherein said cellulose comprises at least about 88% alpha cellulose;  
     (f) treating said extraction liquor with an acid to precipitate lignin contained therein, and  
     (g) separating said lignin from soluble hemicellulose in said extraction liquor.  
     The recovered cellulose component comprises at least 88% alpha cellulose and is useful as a starting material for the production of ethanol.

TECHNICAL FIELD

[0001] The present invention is directed to an economic method for fractionating renewable biomass safely to cleanly separate the cellulosic, hemicellulosic and lignin components of fibrous plant materials in a process without the concomitant production of by-products which could pollute water, land or air. The separated components are useful for the production of fossil fuel derivatives, biodegradable plastics, edible protein, and a variety of other products.

BACKGROUND OF THE INVENTION

[0002] The world today is facing growing burdens caused by overpopulation, depletion of fossil fuels, increasing demands for fuels, pollution of air, water and land, global warming and climate changes, forest cover destruction, and agricultural land loss. Although to some extent some of these concerns can be met through the improved use of solar energy and windpower and increased nuclear power, more conservation of resources and more efficient use of resources are always being sought.

[0003] Fibrous cellulosic material, such as straw, corn stalks (stover), bagasse, hardwoods, cotton stalks, kenaf and hemp, are composed primarily of cellulose (typically, 40-60% dry weight), hemicellulose (typically 20-40% by dry weight) and lignin (typically 5-25% by dry weight). These components, if economically separated fully from one another, can provide vital derivative sources of fermentable sugars for the production of alcohols, ethers, esters and other chemicals. There is a growing interest in the manufacture of biofuels from cellulosic biomass by fermentation with enzymes or yeast. To date, the majority of that interest has focused on the use of starch, cane and beet sugar. As used herein, biofuels refers to fuel (ethanol) for the generation of electricity and for transportation. Biofuels are beneficial in that they add fewer emissions to the atmosphere than petroleum fuels. They also are beneficial in that they use herbaceous and sparsely used woody plants and, particularly, plant wastes that currently have little or no use. Biofuels are obtained from renewable resources and can be produced from domestic, readily available plants and wastes, thus reducing dependence on coal, gas and foreign fossil fuel in addition to boosting local and world-wide economies.

[0004] To date, however, there has not been an economical method for cleanly separating the basic components of fibrous, ligno-cellulosic materials and the fermentable sugars they represent from one another. In particular, it has proved difficult to economically separate the mixed hexose and pentose structured hemicellulose from the lignin and other, minor, components, such as lipids and silica, present in biomass. The processes which exist today focus on techniques such as ball-milling, two-roll milling, cryogenic grinding, explosive depressurization, ultrasonics and osmotic cell rupture followed by ethanol extraction, as well as conventional pulping techniques. All use high levels of technology, fossil energy and investment and, accordingly, are expensive and, often, highly polluting. For example, conventional pulping processes, which use high temperatures (e.g., 175° C.) and pressure (e.g., 175 psi) and sulfite, kraft or alkali to obtain purified cellulose, known as alpha pulp, are well recognized as involving high investment, energy and operating costs, including recovery of chemicals, which are accompanied by severe problems of air and water pollution and the production of toxic materials.

[0005] In spite of these difficulties, interest in finding alternative methods for fractionating cellulosic biomass remains high, since fibrous cellulosic vegetation constitutes a vast renewable potential source of energy from plant sugars as ethanol and of animal food. Worldwide, it is estimated that there are available over twenty billion tons annually of agricultural fibrous waste that could provide a source of cellulose, hemicellulose and lignin. In the U.S., wheat straw and corn stover constitute some billion tons annually, much of which is wasted.

OBJECTS OF THE INVENTION

[0006] Accordingly, it is an object of this invention to provide an economical and efficient method of fractionating a wide variety of renewable fibrous organic biomass to its cellulose, hemicellulose and lignin components which then can be used as base materials for the production of sugars for energy, plastics, chemicals, and protein.

[0007] It is a further object of this invention to help reduce global warming through a reduction in air pollution. Providing a clean and cost efficient process for the production of cellulose makes possible the production of bioethanol in large quantities at an economical cost. Having a means for the efficient production of ethanol using local materials that otherwise would be burned as waste will allow, for the first time, the substitution of ethanol for a significant portion of the enormous amount of gasoline that is used annually. This will reduce air pollution caused by vehicle exhaust, as well as that caused by the emission of nitrogen gases derived from the burning of what conventionally has been considered waste biomass.

[0008] It is a further object of this invention to provide a method of fractionating biomass that is environmentally clean, with no toxic by-products and no toxic air or water waste stream, with a closed liquid and chemical system.

[0009] It also is an object of this invention to provide such a method that can be provided in a wide variety of sizes (e.g., 30-300 tons per day), based on ready availability of raw stock, using relatively simple equipment, low installation costs, low cost chemicals and simple automated technology.

[0010] It further is an object of this invention to provide such a method which results in spent liquors which contain low percentages of nitrogen and which, if not recycled, can be used as fertilizer to provide a slow release source of chemically active and microbially digestible nitrogen to soil.

[0011] Further objects of this invention will be apparent from the description of the invention set forth below.

SUMMARY OF THE INVENTION

[0012] In accordance with the present invention, a method for fractionating fibrous biomass comprising cellulose, hemicellulose and lignin components to separate said lignin, cellulose and hemicellulose comprises:

[0013] (a) shredding said fibrous biomass;

[0014] (b) concurrently with or prior to said shredding, contacting said biomass with an aqueous solution of a nitrate ion source at a concentration of about 0.1-0.3% to initiate nitration of the lignin component of said biomass;

[0015] (c) submerging said partially nitrated biomass in a dilute aqueous solution of said nitrate ion source in the presence of an aluminum compound at atmospheric pressure and a temperature of about 60°-100° C. to complete the nitration of said lignin component;

[0016] (d) contacting the nitrated biomass produced in step (c) with an alkaline extraction liquor comprising NH₄OH at a concentration and temperature sufficient to solubilize and separate said nitrated lignin component and said hemicellulose component from said cellulose component of said biomass;

[0017] (e) recovering said cellulose from said extraction liquor containing said dissolved nitrated lignin and hemicellulose components, wherein said cellulose comprises at least about 88% alpha cellulose;

[0018] (f) treating said extraction liquor to precipitate said solubilized lignin; and

[0019] (g) recovering said precipitated lignin from said extraction liquor containing said solubilized hemicellulose.

[0020] The isolated alpha cellulose, hemicellulose and lignin can be used for the production of a wide variety of products. The alpha cellulose can be used as the starting material for the production of regenerated cellulose products such as rayon and cellophane; water soluble films; cellulose nitrate; and glucose derivatives, including fermentation products, such as ethanol. Hemicellulose can be used for the production of water soluble polymers, butylene glycol and ethanol and other fermentation products. Lignin can be used for the production of polyurethane polyol foams, paper sizing, adhesives, resin extenders, and other products.

[0021] In a preferred embodiment of this invention, the recovered cellulose (hexose) is subjected to a fermentation process to produce bioethanol, a valuable source of fuel and energy. The hemicellulose component, a mixture of hexose and xylose sugars, also can undergo fermentation to produce bioethanol, thus maximizing the production of ethanol.

DETAILED DESCRIPTION OF THE INVENTION

[0022] The process of the present invention presents an efficient and clean procedure for the fractionation of the three basic components of plant life—cellulose, hemicellulose and lignin—from renewable, fibrous biomass, for use, individually or in combination, in the manufacture of such products as ethanol fuel, biodegradable plastic, oil, gas and coal derivatives, fertilizer, pharmaceutical and medical products, and adhesives, among others.

[0023] The process of this invention is a specific modification of a process used for making paper pulp for the manufacture of linerboard described in U.S. Pat. No. 4,652,341. It now has been found that by suitable and precise preparation of the biomass and specific selection of the choice of acid and alkali agents and concentrations and control of the processing pH throughout the process, the cellulose, hemicellulose and lignin components can be cleanly and efficiently separated from one another and that at least about 88% of the cellulose component can be provided in the form of alpha cellulose. A cellulose product containing this high a percentage of alpha cellulose is unsuitable, i.e., too weak, to be used for making paper, but this amount of alpha cellulose is a necessary minimum as a starting material for producing bioethanol. In addition, almost all of the hemicellulose is retained with the insoluble lignin as soluble sugars (xylose and hexose) when the cellulose component is isolated, and the hemicellulose can be easily separated from the lignin such that it, too, can provide a source for bioethanol production.

[0024] In accordance with the process of the invention, the lignin component of the biomass is nitrated and subsequently extracted in a procedure in which an insoluble lignin nitrate is produced by treating the biomass with an acid at a pH of about 1-6, followed by cooking at a temperature within the range of about 85-100° C. with an alkali in the range of about 8-11.5 to dissolve the nitrated lignin and the hemicellulose, thereby dissociating the cellulosic fraction from the rest of the biomass. This is followed by isolation of the cellulose, at least about 88% of which is in the form of alpha cellulose, and then a second acidification of the used alkaline liquor remaining from the separation of the cellulose to precipitate and separate the lignin nitrate from the solubilized hemicellulose-containing liquor. The hemicellulose then is recovered from the extraction liquor. The hemicellulose solution desirably contains a minimum of about 5% concentration of hemicellulose sugars.

[0025] Because the isolated cellulose component comprises at least about 88% alpha cellulose, which is about 10% more alpha cellulose than found in paper-making pulp, it is highly suitable as a starting material for the production of ethanol.

[0026] As used herein, the term “renewable, fibrous biomass,” “fibrous, ligno-cellulose biomass” or, simply, “biomass,” refers to herbaceous and woody plants, agricultural and forestry wastes and residues, and crops grown solely for energy purposes. Agricultural waste includes agricultural residues, such as wheat, oat, rice and barley straw, corn stover, sugar cane, bagasse, shrubs and forest trimmings. Forestry wastes include logging residues and non-commercial type trees (such as poplar, alder and cottonwood) that need to be thinned from forests or from undeveloped land that is to be developed. Energy crops include fast-growing trees, especially short-fibered species, such as poplars, willows, alders, etc.

[0027] All forms of fibrous biomass can be used in the process of the present invention, including, without limitation, woody materials, such as U.S. southern pine, spruce, beech, bamboo, and poplar, and grassy materials, such as straws, bagasse and kenaf. In addition, high yield shrub growth, such as coppice willow, and agricultural wastes, such as cotton stalks, sugar cane bagasse and corn stalks, can be used. In the process of the invention, it is possible to separate bark from useable cellulose fiber, facilitating the use of waste forest trimmings and shrubs by interruption of the nitrating process and separation of the bark by flotation.

[0028] An important first step in the process is correct fiber preparation. The biomass should be in a small-sized particulate form. It is important that the biomass not only be cut into small pieces that are relatively uniform, but that the pieces be shredded or flaked into homogeneous longitudinal fiber structures, rather than simply chopped. This will enhance penetration of the biomass particles by maximizing the amount of internal surface area that will be directly exposed to the nitrating agent, thus accelerating nitration of the lignin, while maintaining the fibrous structure. Desirably, the biomass pieces are cut into approximately two inch lengths and then shredded or crushed or flaked. Hereinafter, the biomass will be referred to as “shredded;” it is to be understood that this term encompasses “crushed” and “flaked” as well.

[0029] Either before or concurrently with the initial cutting step, the biomass desirably is washed to eliminate dirt, metals and other debris that may be present.

[0030] There obviously can be significant physical differences among the various sources of possible starting materials. These can be minimized through the addition of a low acid percent liquor to the biomass being shredded. The low acid liquor typically is added in an amount that is approximately 20-30% by weight of the dry biomass being shredded, although this can vary depending upon the specific biomass.

[0031] Specifically, the shredded biomass is sprayed with a solution of the nitrating agent so as to maintain the fibrous structure of the biomass while initiating nitrating action of the lignin. This pre-treatment step desirably is carried out concurrently with the shredding of the biomass to facilitate the shredding as well as to initiate nitration of the lignin, but it also can be carried out subsequent to the shredding. A preferred nitrating agent is HNO₃ within the range of about 0.1% to about 0.3%, preferably about 0.2-0.3%. Desirably, the nitrating solution has a temperature in the range of about 60-80° C.

[0032] This pretreatment step is valuable, as it helps maintain the fiber structure of the shredded biomass (the fiber bundles), which, in turn, helps maintain the efficiency of subsequent washing and drainage steps and helps reduce loss of fines, thereby increasing yield, as well as ultimate chemical and water usage. In addition, it makes the shredding process easier and more consistent, as the acid acts as a lubricant to the mechanical process.

[0033] The acid-sprayed biomass then is introduced into a digester containing a solution of nitrating agent. The preferred nitrating agent is HNO_(3.)

[0034] The nitrating agent in the digester generally is provided in solution at a concentration within the range of about 1% to about 6%, depending upon the nature of the biomass used as a starting material; lower concentrations generally are sufficient for biomass such as straw; more solid biomass such as shredded wood chips typically require higher concentrations. Desirably, the concentration of the nitrating agent is within the range of about 1.0% to about 1.5% for straw or grass, within the range of about 1.5% to about 2.5% for biomass such as corn stover, hemp and kenaf, within the range of about 3.0% to about 4.0% for shredded brush or hardwood, and within the range of about 5.0% to about 6.0% for shredded coniferous wood. These ranges are intended as guidelines; the exact percentage used in a given process will be dependent upon the type of fibrous biomass being treated, the type of digester being used, the temperature at which the nitration is carried out, any time limitations and the possible use of false pressure with gas recycling.

[0035] The amount of the solution of nitrating agent used in the nitration step should be sufficient to completely submerge or contact by mild agitation and recirculation the particulate fibrous biomass and adequate, on a stoichiometric basis, to nitrate substantially all of the readily accessible lignin content of the raw material. Providing the biomass has been properly shredded, this usually will mean, for example, the formation of a mixture of about 5-6 parts of liquid nitrating agent to about 1 part of dry biomass. Preparation of the biomass is of utmost importance in achieving a closed water system and access to lignin.

[0036] The nitration step can be carried out at a temperature between 75 and 100° C. Desirably, the step is carried out at a temperature which is sufficient to effect nitration of the lignin but is not so high (over about 85-90° C.) as to cause gas formation which reduces the nitrating capability unless provisions are made to recycle the gas. Below about 75° C. the reaction takes an excessive amount of time. Temperatures within the range of about 80° C. to about 85° C. are preferred if gas and pressure formation are not desired. Additionally, care must be taken with regard to the manner of heating, so that the heat is applied in such a way that hot spot temperatures, which release gas, are avoided.

[0037] To reduce the nitrating processing time it is desirable to include an aluminum-containing accelerator in the nitrating solution. A preferred accelerator is Al₂(SO₄)₃-18H₂O, commonly known as alum. The accelerator serves to shorten the time needed to convert the lignin component of the biomass to an insoluble lignin nitrate.

[0038] Generally, the amount of the alum required is at least about 0.065 parts, preferably 0.065 to 0.105 parts, by weight of per each part of active HNO₃.

[0039] The nitrating step, as noted above, can be carried out at a relatively mild temperature, i.e., between about 75° C. and 100° C., preferably within the range of about 80° C. to about 85° C. The process can be carried out at atmospheric pressure, or, if desired, at a pressure within the range of about 10 to about 35 p.s.i.g. if it is desired to further shorten the reaction time, provided that the gas produced is recycled constantly into the acid nitrating liquor and biomass.

[0040] Regardless of whether a light pressure is employed, it is desired that any gases not be vented. With the gases retained in, and in contact with the fibrous biomass in, the treating vessel, a false pressure is created, permitting the temperature for the nitration step to be kept at or below about 100° C. The degree of false pressure created is dependent upon the concentration of the nitration agent and a temperature above 85° C.

[0041] By using light or only atmospheric pressure, temperatures within the range of about 75° C.-about 85° C. and a nitration agent having a concentration as described above, the nitration period for shredded straw generally is within the range of about 30 to about 45 minutes, about 45 to about 60 minutes for shredded brush or hardwood and about 90 to about 180 minutes for shredded coniferous wood chips.

[0042] Cooking vessels should be spherical and continuous type digesters. Conventional batch digesters should be avoided because their explosive emptying process destroys the physical fibrous structure which should be maintained to facilitate washing and draining of the biomass.

[0043] Once the nitrating step has been completed, the nitrating agent is pressure drained from the fibrous biomass and the nitrated biomass is washed at least once with water to remove surface chemical. The wash water preferably is hot, e.g., at a temperature of about 75°-100° C., and is used in minimal amounts. Sufficient washing is indicated when the pH of the wash water is within the range of 5-6, preferably about 6. Failure to achieve this pH level indicates poor pretreatment of the biomass or too much acid and will require undesirably high amounts of alkali in the subsequent step of the biomass treatment process. The separated nitrating agent advantageously can be used over again by refreshing it through the addition of further nitrating agent and alum source as needed and then recycled to serve as nitrating agent for further biomass cooking. By recycling the liquor in this way, heat and chemicals are conserved. The acidic wash water also desirably is recycled, such as for use in the pretreatment of a new batch of biomass or to adjust the pH value of the subsequent alkali stage black liquor.

[0044] The nitrated and washed biomass then is contacted with an alkaline extraction liquor. The biomass is submerged in the extraction liquor. The lignin nitrate is readily dissolved in basic aqueous media, i.e., that having a pH of about 7.5 to about 12. The alkaline concentration of the extraction solution must be sufficient for the solution to leach out, i.e., dissolve, most, if not essentially all, of the nitrated ligneous material in the fibrous biomass. The hemicellulose, silica and lipids also are extracted from the cellulose in this step. A highly preferred alkaline extraction liquor comprises NH₄OH as the alkaline agent. The pH of the alkaline extraction liquor preferably is within the range of about 11 to about 12.

[0045] The amount of alkaline extraction liquor used should be sufficient to completely submerge or continuously contact the nitrated biomass so as to dissolve out and effectively separate the nitrated lignin, hemicellulose and other, minor, components of the biomass. A typical wt:wt ratio of extraction liquor to nitrated biomass is about 10:1. The temperature at which the alkaline extraction step is conducted should be sufficient to aid the dissolution and separation of the nitrated ligneous material. Preferably, a temperature of about 80° to about 100° C., preferably 80° to about 90° C., is used. The alkaline extraction step desirably is carried out at atmospheric pressure.

[0046] The time required to effect dissolution of the nitrated lignin and hemicellulose depends upon the initial strength of the alkaline extraction liquor, the type of fibrous biomass being treated and the temperature at which the extraction is conducted. Generally, however, the time necessary for the extraction ranges between 30 and 60 minutes, preferably about 60 minutes.

[0047] Following treatment with the alkaline extraction liquor, the cellulose is separated from the extraction liquor containing the now-solubilized nitrated lignin and the hemicellulose sugars. This can be accomplished by draining the liquor, such as by passing the biomass through a high density press, and washing the cellulosic pulp. Each of the cellulosic pulp and the wash water should have a pH within the range of about 8-9, preferably about 8, after removal of the wash water. Failure to reach this pH level is indicative of inadequate washing and will require pH adjustment through the addition of acid.

[0048] The resultant separated cellulose comprises at least about 88%, preferably at least 90% or 95% alpha cellulose. The total of the cellulosic fiber, known as holo cellulose, is a mixture of the 100% carbohydrate alpha cellulose pulp and residual silica, pentosans, polyuronides, low molecular weight hexosans and other non-cellulosic residues. These non-alpha cellulose components are not desirable in the production of ethanol. Desirably, these components are held to less than 5% of the cellulosic product recovered.

[0049] The alkali extraction liquor containing the dissolved lignin and soluble sugars from hemicellulose from which the cellulose has been removed will be referred to herein as “black” liquor. The black liquor has a pH in the range of about 8.0 to about 8.5. The ligneous and non-cellulosic sugar components of the black liquor can be recovered by acidifying the black liquor, preferably to a pH within the range of about 5 to about 6, to precipitate the lignin and silica which then can be concentrated by centrifugation, leaving the hemicellulosic sugars in solution and at the correct pH for fermentation. Typically, the hemicellulose sugars are at a 5% plus concentration.

[0050] Because both the acid and alkali agents used in the process described above can be recycled and used in subsequent batches, the process can be carried out in a closed liquid system, thereby minimizing chemical and water usage and eliminating air and water pollution. In the event of an emergency that would necessitate disposal of the nitrating agent and/or the alkali extraction liquor, they would be accepted by urban sewage operations or, in the case of the black liquor, be found acceptable for use as fertilizer. Further advantages of the process include using readily available and inexpensive chemicals. Equipment for carrying out the process can be installed at far less cost (80%) than existing and presently proposed alternative methods, and operational costs are also less than those of known alternatives.

[0051] To obtain the desired products of the above-described process, it is necessary for the end point of each step of the process to reach the indicated pH. These pH levels also are required to obtain maximum chemical usage and to allow the use of a closed reaction system, i.e., a system in which chemical recovery is not required.

[0052] A further advantage of the present process is that it can be computer controlled. For any desired type of biomass, once initial standards for precise chemical concentration, time, temperature, pressure (if the latter is desired) and end point available fermentable sugars within the guidelines given above have been determined, the process can be controlled by computer, with the single exception of an initial test of drainage capability and residual acidity of the first stage nitrating action.

[0053] As noted above, the isolated cellulose, hemicellulose and lignin components of the biomass are useful starting materials for the production of many types of useful products. In a preferred embodiment of this invention, the cellulose is used for the production of ethanol. The alpha cellulose is converted to simple glucose sugars and enzymes ferment the glucose to ethanol in accordance with any of the conventional procedures known to persons of skill in the art. The sugars of the hemicellulose component, predominately xylose with hexose, also can be obtained and subjected to fermentation to convert the sugars to ethanol using procedures known to those of skill in the art. Certain enzymes can react with both the cellulose sugars and hemicellulose sugars.

[0054] In addition, the cellulose, hemicellulose and lignin components can be used as starting materials for the production of numerous other types of products, as known to those of skill in the art. Because the components can be isolated from one another in a simple, clean and efficient manner for the first time by the process described above, they can be provided economically as substitutes for derivatives of fossil fuels. Examples of other products from cellulose include regenerated cellulose products, such as rayon, cellophane, cellulose acetate, molded plastics, apparel fibers, biodegradable films and laminated films. The cellulose also can be used to make water soluble films, such as carboxy methyl cellulose, detergents, and cellulose polyacrylonitrate. The cellulose also can be used to provide other glucose derivatives, including fructose, sorbitol, gluconic acid, citric acid, and glycerol. The hemicellulose sugars can be used to make a variety of water soluble polymers, including cellulose polyacrylonitrate, furfural, xylitol glycerol, and butylene glycol. The lignin component can be used to make vanillin, polyurethane polyol foams, and as a component in paper sizing, adhesives, stiffeners and resin extenders. It also can be used as a binder in a wide variety of products, including asphalt, concrete, and animal feed pellets, as a stabilizer in a wide variety of emulsions and in boiler feed water, as a dispersant in oil well drilling needs and in industrial cleaners, and in the leather tanning industry to enrich color by enhancing the speed of dye penetration and to enhance water resistance. Lignin also can be used as an extender in a wide variety of products, including paints, laminating resins, phenolic molding compounds, fiberglass, adhesives and non-woven fabric binders and coatings. Other uses for lignin are known to persons of skill in the art.

[0055] In addition, although, as noted above, the black liquor can be renewed and recycled for use as an extraction liquor for subsequent batches of nitrated biomass, alternatively, if desired, it can be used as a fertilizer to provide a slow release source of chemically active and microbially digestible nitrogen to soil.

[0056] The invention is further illustrated by the following example, which is not to be construed as limiting.

EXAMPLE

[0057] In accordance with the present invention, rice straw was fractionated. 500 g. of rice straw were chopped into 2 inch lengths which then were washed to eliminate dirt and other debris, then the chopped pieces of straw were shredded. During the shredding of the straw twice through a disk refiner it was sprayed with a 200 cc solution of 0.20% HNO₃.

[0058] The pretreated straw, moistened and in small. particulate fiber bundles, was introduced into a spherical digester containing an 80° C. solution of 5 L of 1.5% HNO₃ and 0.15% of AL₂(SO₄)₃18H₂O. The nitration of the lignin in the straw was carried out at 80°-95° C. for 30 minutes at atmospheric pressure. The rise in temperature and transient false pressure created were the result of the chemical reaction occurring.

[0059] At the conclusion of the nitration step, the nitrating solution was drained from the digester and the pressed nitrated straw was washed with one liter of water having a temperature of 80° C. twice. The wash water then was drained from the straw.

[0060] The nitrated and washed straw was submerged in 5.0 L of water and NH₄OH was added to a pH of 11.5. The alkaline extraction step was conducted at a temperature of 100° C. for a period of 30 minutes. The biomass then was press drained to remove the alkaline extraction liquor, thereby separating the cellulose from the hemicellulose and lignin portions of the biomass. The cellulose fraction then was washed with 2 L of wash water having a temperature of 80° C. The cellulosic fraction had an O.D. yield of 40.4% and was 96% holo cellulose, 88% of which was alpha cellulose.

[0061] The black liquor drained from the cellulose was acidified through the addition of sufficient HNO₃ to a pH of 6 to precipitate the lignin. The liquor then was centrifuged to separate and concentrate the lignin. The residual liquor contained the hemicellulose sugars comprising 6% of the solution. 

1. A method for fractionating fibrous biomass comprising cellulose, hemicellulose and lignin components to separate said lignin, cellulose and hemicellulose from one another comprises: (a) shredding said fibrous biomass; (b) concurrently with or prior to said shredding, contacting said biomass with an aqueous solution of a nitrate ion source at a concentration of about 0.1-0.3% at a temperature in the range of about 60° to about 80° C. to initiate nitration of the lignin component of said biomass; (c) submerging said partially nitrated biomass in an aqueous solution of a nitrate ion source in the presence of an aluminum compound at a temperature within the range of about 75-100° C. for a time sufficient to complete the nitration of said lignin component; (d) contacting the nitrated biomass produced in step (c) with an alkaline extraction liquor comprising NH₄OH at an initial concentration sufficient to solubilize said nitrated lignin component and said hemicellulose component from said cellulose component of said biomass; (e) recovering said cellulose from said extraction liquor containing said solubilized nitrated lignin and hemicellulose components, wherein said cellulose comprises at least about 88% alpha cellulose; (f) treating said extraction liquor with an acid to precipitate lignin contained therein, and (g) separating said lignin from soluble hemicellulose in said extraction liquor.
 2. The method of claim 1, wherein said biomass comprises woody material or grassy or straw material.
 3. The method of claim 1, wherein said nitrate ion source used to initiate nitration of said lignin in step (b) comprises HNO_(3.)
 4. The method of claim 3, wherein said solution of said nitrate ion source in step (c) comprises about 1% to about 6% HNO₃.
 5. The method of claim 4, wherein said biomass is contacted with said solution of said nitrate ion source in step (c) for about 30 to 180 minutes.
 6. The method of claim 4, wherein said biomass comprises straw or grass and said solution of said nitrate ion source comprises about 1.0% to about 1.5% HNO₃.
 7. The method of claim 4, wherein said biomass comprises corn stover, hemp or kenaf and said solution of said nitrate ion source comprises about 2.0% to about 2.5% HNO₃.
 8. The method of claim 4, wherein said biomass comprises shredded brush or hardwood and said dilute solution of said nitrate ion source comprises about 3.0% to about 4.0% HNO₃.
 9. The method of claim 4, wherein said biomass comprises shredded coniferous wood and said solution of said nitrate ion source comprises about 5.0% to about 6.0% HNO₃.
 10. The method of claim 1, wherein said aluminum compound comprises alum.
 11. The method of claim 1, wherein said nitrated biomass is washed prior to contacting it with said alkaline extraction liquor.
 12. The method of claim 1, wherein said alkaline extraction liquor has a pH within the range of about 11 to 12 following contact with said nitrated biomass.
 13. The method of claim 12, wherein said extraction step is carried out at a temperature within the range of about 80° and about 100° C.
 14. The method of claim 1 wherein said nitrated biomass is contacted with said alkaline extraction liquor for a period of about 30-60 minutes.
 15. The method of claim 1, wherein said cellulose separated from said extraction liquor comprises at least 90% alpha cellulose.
 16. The method of claim 15, wherein said cellulose separated from said extraction liquor comprises at least about 93% alpha cellulose.
 17. The method of claim 1, wherein said extraction liquor in step (f) is treated with HNO₃ to precipitate said lignin.
 18. The method of claim 17, wherein said extraction liquor is acidified to a pH of about 5.0 to about
 6. 19. A method for fractionating fibrous biomass comprising cellulose, hemicellulose and lignin components to separate said lignin, cellulose and hemicellulose from one another comprises: (a) shredding said fibrous biomass; (b) concurrently with or subsequent to said shredding, contacting said biomass with an aqueous solution of HNO₃ at a concentration of about 0.1-0.3% to initiate nitration of the lignin component of said biomass; (c) submerging said partially nitrated biomass in an aqueous solution of about 1% to about 6% HNO₃ in the presence of alum at a temperature in the range of about 75-100° C. for a time sufficient to complete the nitration of said lignin component; (d) washing said nitrated biomass; (e) submerging the nitrated biomass produced in step (d) in an alkaline extraction liquor comprising NH₄OH at a concentration sufficient to extract said nitrated lignin component and said hemicellulose component from said cellulose component of said biomass; (f) recovering said cellulose from said extraction liquor containing said nitrated lignin and hemicellulose components, wherein said cellulose comprises at least about 88% alpha cellulose; (g) treating said extraction liquor with acid to precipitate said lignin contained therein, and (h) separating said lignin from said hemicellulose-containing liquor.
 20. A method of obtaining cellulose from fibrous biomass comprising cellulose, hemicellulose and lignin components which comprises: (a) shredding said fibrous biomass; (b) concurrently with or prior to said shredding, contacting said biomass with an aqueous solution of a nitrating ion source at a concentration of about 0.1-0.3% to initiate nitration of the lignin component of said biomass; (c) submerging said partially nitrated biomass in an aqueous solution of a nitrate ion source in the presence of an aluminum compound at a temperature of less than about 100° C. for a time sufficient to complete the nitration of said lignin component; (d) contacting the nitrated biomass produced in step (c) with an alkaline extraction liquor comprising NH₄OH in an amount sufficient to extract said nitrated lignin component and said hemicellulose component from said cellulose component of said biomass; and (e) recovering said cellulose from said extraction liquor containing said nitrated lignin and hemicellulose components, wherein said cellulose comprises at least about 88% alpha cellulose.
 21. The method of claim 20, wherein said nitrate ion source used to initiate nitration of said lignin in step (b) comprises HNO₃.
 22. The method of claim 20, wherein said solution of said nitrate ion source in step (c) comprises about 1% to about 6% HNO.
 23. The method of claim 22, wherein said biomass is contacted with said solution of said nitrate ion source in step (c) for about 30 to 180 minutes.
 24. The method of claim 20, wherein said aluminum compound comprises alum.
 25. The method of claim 20, wherein said alkaline extraction liquor has a pH within the range of about 11 to 12 following contact with said nitrated biomass.
 26. The method of claim 20, wherein said extraction step is carried out at a temperature within the range of about 80° to about 100° C.
 27. The method of claim 20, wherein said cellulose comprises at least about 90% alpha cellulose.
 28. A method of obtaining ethanol from fibrous biomass comprising cellulose, hemicellulose and lignin components which comprises: (a) shredding said fibrous biomass; (b) concurrently with or prior to said shredding, contacting said biomass with an aqueous solution of a nitrating ion source at a concentration of about 0.1-0.3% to initiate nitration of the lignin component of said biomass; (c) submerging said partially nitrated biomass in an aqueous solution of said nitrate ion source in the presence of an aluminum compound at a temperature of less than about 100° C. for a time sufficient to complete the nitration of said lignin component; (d) contacting the nitrated biomass produced in step (c) with an alkaline extraction liquor comprising NH₄OH in an amount sufficient to extract said nitrated lignin component and said hemicellulose component from said cellulose component of said biomass; (e) recovering said cellulose from said extraction liquor containing said nitrated lignin and hemicellulose components, wherein said cellulose comprises at least about 88% alpha cellulose; (f) washing said recovered cellulose; (g) converting said cellulose into glucose sugars and (h) fermenting said glucose sugars to produce ethanol. 